This invention relates to magnetic particles used to make magnetic recording media. It is shown that particles having a nearly spherical or equiaxed shape have many important advantages over the acicular or platelet shaped conventional particles.
There are two quite different particle shapes used in conventional particulate magnetic recording media. The most often used particles (e.g. iron oxide (Fe2O3), metallic iron (Fe) and chromium dioxide (CrO2)), are highly acicular or cigar shaped. The energetically preferred or xe2x80x9cmagnetic easy axisxe2x80x9d of these acicular particles is parallel to the length of the particle. The magnetic characteristics arise from the xe2x80x9cshape anisotropy energyxe2x80x9d and are a first order function of the length to diameter ratio of the particles.
The other type of particles used, such as Barium Ferrite or Strontium Ferrite (BaFe and SrFe),are platelet or tabular in shape, with the easy axis perpendicular to the plane of the platelet. Their magnetic characteristics arise primarily from internal xe2x80x9ccrystalline anisotropyxe2x80x9d, and are only a second order function of the size or aspect ratio of the particle.
Regardless of the shape of the particle, for particulate recording media, optimal conventional longitudinal recording performance is obtained when the easy axes of the particles are oriented parallel to the media travel direction.
Coating Shear Anisotropy
The magnetic layer of conventional particulate recording media is deposited from a liquid dispersion by some type of web coating technique. The viscous fluid shear involved in this process, and the particle shape, generate torque on the particles that results in at least a moderate degree of particle orientation. This produces a magnetic anisotropy in the coated layer.
In the case where acicular particles are being used to make magnetic tape, the resulting minimum magnetic energy or xe2x80x9ceasyxe2x80x9d axis orientation is parallel to the tape motion. Hence, the shear orientation makes the desired longitudinal magnetic orientation of the tape somewhat easier to accomplish.
In the case where acicular particles are being used to make floppy disks, the shear orientation results in a uniaxial, rather than the desired circumferential, magnetic anisotropy in the plane of the disk. This produces an undesirable twice-around modulation in the magnetic properties of the disk. Special magnet assemblies are employed to disorient the particles after coating. This is done in an effort to make the magnetic layer more isotropic in the plane of the disk.
When the platelet shaped BaFe particle is being web coated, the viscous shear torques cause the particle magnetic easy axes to be at least partly oriented perpendicular to the plane of the coated layer. In conventional longitudinal recording applications, this results in signal amplitude loss and distortion. To solve this problem when coating BaFe tape, an orienting magnetic field is applied to cause the particle easy axes to align parallel with the tape length.
In making web coated floppy disks with BaFe, using an orienting field is not possible. There is no twice around modulation of the magnetic properties, but the signal amplitude loss and distortion, resulting from the vertical shear orientation, limit the performance of BaFe floppy disks. Disorienting magnets similar to those discussed above are sometimes used to minimize this problem.
Particle Shape and the Dispersion Process
By their magnetic nature, magnetic particles tend to form together in clumps. These particle clumps result in an important source of noise in the recording layer and hence the powders must be well dispersed prior to coating. The dispersion is typically carried out in some form of high shear ball mill. This process subjects the particles to considerable impact, which tends to cause particle breakage, since the tiny crystalline particles are rather fragile. Broken particles have very inferior magnetic properties, which severely degrade the performance of the magnetic layer. The dispersion process with conventional acicular or platelet particles is always a tradeoff between the dispersion quality (lack of clumps) and the number of broken particles.
Because the magnetic easy axes of the BaFe particles are perpendicular to the plane of the platelet, they tend to clump together in stacks like poker chips. This large area of contact makes them very difficult to separate without breakage and leads to a less than optimum dispersion or many broken particles having degraded magnetic properties.
Acicular particles tend to form pairs of particles, oriented in opposite directions, which have at most a line contact between the pairs. They are thus somewhat easier to separate than the platelet shaped BaFe. On the other hand, the long thin shape of the acicular particles makes them more prone to breakage, and the magnetic characteristics of this type of particle degrade much more quickly as particle breakage occurs. This is because the magnetic hardness of acicular particles is determined by their length to diameter ratio.
Extruded or Co-extruded All-Magnetic Card or Co-extruded Media and Shear Orientation
A new all-magnetic card, described in copending applications listed above, utilizes high coercivity BaFe particles uniformly distributed throughout the volume of the card. The card is useful as a credit card, as well as many other possible applications, and can be recorded anywhere on the surface of the card, as well as on the card edges. The high coercivity BaFe particles may also be used in other applications, such as co-extruded resin for photographic paper, photographic film, inkjet media, or thermal diffusion dye transfer media. Particularly useful embodiments are the use of high coercivity BaFe particles in imaging supports such as those described in U.S. Ser. No. 08/862,703, filed May 23, 1997, entitled xe2x80x9cComposite Photographic Material with Laminated Biaxially Oriented Polyolefin Sheet,xe2x80x9d by R. Bourdelais et al.; U.S. Ser. No. 08/862,234, filed May 23, 1997, entitled xe2x80x9cPhotographic Element with Indicia on Oriented Polymer Back Sheet,xe2x80x9d by R. Bourdelais et al.; U.S. Ser. No. 08/862,901, filed May 23, 1997, entitled xe2x80x9cPhotographic Element with Bonding Layer on Oriented Sheet,xe2x80x9d by R. Bourdelais et al.; U.S. Ser. No. 09/154,881, filed Sep. 17, 1998, entitled xe2x80x9cPhotographic Transmission Display Materials with Biaxially Oriented Polyolefin Sheet,xe2x80x9d by P. Aylward et al.; U.S. Ser. No. 09/154,900, filed Sep. 17, 1998, entitled xe2x80x9cTranslucent Display Paper with Biaxially Oriented Polyolefin Sheets,xe2x80x9d by P. Aylward et al.; and U.S. Ser. No. 09/154,692, filed Sep. 17, 1998, entitled xe2x80x9cTransmission Imaging Display Material with Biaxially Oriented Polyolefin Sheet,xe2x80x9d by P. Aylward et al.
The recording medium may be co-extruded as one layer or as a multilayer structure that can be employed as a photographic element. For example, it could form a recording layer on a photographic image support on the side opposite the imaging layers. Such supports may be used to support images made by processes such as silver halide photography, inkjet, thermal diffusion dye transfer, and the like.
The high coercivity BaFe particles used in the all magnetic card have the conventional hexagonal platelet shape. The standard thermoplastic extrusion process used to form the cards, paper, film or other media involves very high viscous shear and considerable stretching. These, together with the platelet shape of the particle, result in the particles being almost completely oriented with the plane of the platelets parallel to the surface of the card. As in the case of the web coated BaFe layers, this orients the BaFe particle easy axes perpendicular to the plane of the card, and the result is considerable signal amplitude loss and distortion.
The same tradeoff between dispersion quality and magnetic degradation due to particle breakage discussed above, in connection with coated magnetic layers, also occurs with the all-magnetic card or other imaging supports mentioned above. The magnetic properties of BaFe are a second order function of particle size, but breakage caused by excessive milling gives rise to crystal defects in the particle fragments. These defects severely degrade the magnetic properties of the all magnetic card. The coercivity and remanent magnetization are dramatically reduced.
The prior art solution to the vertical orientation problem is to apply an orienting magnctic field during the extrusion process. The extrusion occurs at a temperature of 240xc2x0 C. to 280xc2x0 C. or more. At these temperatures, the magnetic moment and anisotropy of BaFe are reduced from their room temperature values. The result is a lower than desired magnetic torque per unit aligning field.
The field is supplied by a multiturn coil located around the exit slot of the extrusion device. Practical aspects of coil and power supply design result in a maximum aligning field of a few hundred Oe. This relatively low field and reduced magnetic moment limit the aligning torque Even at 280xc2x0 C., the plastic viscosity is rather high, hence the particle rotation rate per unit torque is small. The optimum extruded plastic flow rate can be as high as several inches per second. The length of the aligning field coil is limited by mechanical details. Hence the path length of the region of high orienting field is short. These facts limit the time available to orient the particles.
At high flow rates, and ideal extrusion conditions needed for economical and quality card or imaging support production, the low aligning field and short alignment time, taken together with the low magnetic moment of the particle at the 280xc2x0 C. temperature, result in less than optimum particle orientation parallel to the length of the card or imaging support.
Stretching involved in the extrusion process also tends to orient the BaFe particles in the perpendicular direction. This stretching occurs in zero field regions after extrusion has occurred and tends to counteract the longitudinal orientation created by the aligning field.
According to the present invention, there is provided a solution to the above listed problems of the prior art.
According to a feature of the present invention, there is provided a spherical or substantially spherical magnetic particle for the production of high quality magnetic recording media.
The invention has the following advantages.
1. The spherical or equiaxed particle will not undergo shear orientation during standard magnetic media coating processes, or during all magnetic card extrusion. The media will be magnetically isotropic as fabricated.
2. Because of Item 1, the as fabricated magnetic media will exhibit minimal signal reduction and distortion or twice around magnetic property modulation. The magnetic media will not require magnetic orientation. This will make the media production process much more efficient and inexpensive.
3. Because of the round shape of the particle, the viscous resistance to magnetic orientation will be reduced and magnetic orientation, if desired, can be accomplished more effectively with a lower magnetic field.
4. Because of the point contact between the spherical particles, they will be much easier to disperse than prior art particles.
5. The spherical or equiaxed particle will be very resistant to breakage during dispersion.
6. Because of Items 4 and 5, with the spherical or equiaxed particle of the invention, it will be possible to make a very well dispersed and hence low noise magnetic recording medium having ideal magnetic properties.